Solid state lasers made from Group III-V nitrides such as InAlGaN, have great potential in applications in many areas such as high-resolution full-color printing, high-density optical storage, advanced display systems, and optical communications. Since materials in the Group III-V nitride system have bandgaps that vary from 1.9 eV to 6.2 eV, a laser or light emitting diode ("LED") made from a Group III-V nitride may emit light at a wavelength ranging from 380 to 600 nanometers (nm). A laser made from GaN can emit light in the ultra-violet region of the spectrum whereas a laser made from InN can emit light in the red region. Therefore, the Group III-V nitride material system can produce lasers at wavelengths covering a wide range of the electromagnetic spectrum.
The lasers formed by materials in the Group III-V nitride system which produce emissions at short wavelength such as green or blue have particularly significant implications in high-resolution printing and high-density optical storage. For instance, substantial effort has been expended in developing a blue laser for high-resolution printing because of a blue laser's small fundamental spot size and superior depth of focus. The small spot size allows for higher "dpi" (dots per inch) printing, and therefore improved resolution. The superior depth of focus enables a blue laser to be easily focused, allowing for the use of relatively low cost optics even in a high resolution printing system.
Short-wavelength lasers also allow for high density optical storage of audio information, video information, and data. Conventional compact disc ("CD") players and CD-ROM drives use a infrared laser beam. Currently, a standard 12.7 cm CD can hold up to 650 megabytes of data. Blue lasers, at half the wavelength of red lasers, have much smaller spot sizes and can read and write in finer detail on optical discs, thereby substantially increasing their data capacity. They will allow audio CD's and CD-ROMS to store up to at least five times the information they currently hold.
As stated previously, in addition to producing short-wavelength lasers, the Group III-V nitride material system can produce lasers which emit light at a wavelength ranging from 380 to 600 nanometers (nm). The ability to produce lasers with large wavelength separation with a single material system is important in full-color xerographic printing. Generally, in xerographic printing, a full-color print is formed by superimposing an electrostatic image in black with an electrostatic image in each of the three primary colors (cyan, magenta, and yellow). In one architecture, to achieve high-speed single-pass printing, four laser beams are typically required, one for each color and black. The laser beams simultaneously strike a single raster output polygon mirror and a single set of scan optics. The beams are then separated by optical filters, and each beam is directed to a photoreceptor for printing a color. A similar application is to use red, green, and blue lasers to make full-color film printing for computer generated movies.
In order for the optical filters to effectively isolate each beam at a reasonable cost, the laser beams must typically have a separation of at least 50 nanometers in their wavelengths. To make such a four-laser array on the same substrate, the use of two semiconductor material systems is generally required. For instance, the Al.sub.x Ga.sub.1-x As material system can produce lasers with wavelengths from approximately 750 nanometers to 850 nanometers whereas the AlGaInP material system can produce lasers from approximately 630 to 700 nanometers. In contrast, a laser array formed using materials in the Group III-V nitride system can achieve large wavelength separations without using a second material system.
Red, green, and blue lasers can also be used to produce brighter and more efficient projection displays. They can also be used for backlighting in direct view displays such as screens for laptop computers. Furthermore, due to the physical and electronic properties of materials in the Group III-V nitride system, devices formed using such materials can withstand higher temperatures, higher power densities, and harsher environmental conditions. Hence, the need for lasers formed using Group III-V nitrides crosses a wide spectrum of applications.
Unfortunately the development of lasers formed by Group III-V nitride materials has been hampered by numerous problems in the processing technology of such materials. For instance, the difficulties in forming high-quality single-crystalline Group III-V nitride materials over large areas are well known. Group III-V materials tend to form dislocations and cracks easily. They are also difficult to remove by etching. Due to the difficulty in etching III-V nitride materials, techniques to selectively deposit nitride materials over a smaller area have been developed. An example of such a selective deposition technique is described in "Fabrication of GaN Hexagonal Pyramids on Dot-Patterned GaN/Sapphire Substrate via Selective Metalorganic Vapor Phase Epitaxy" by Kitamura et al., Jpn. J. App. Phys. vol. 34(1995) pp. L1184-1186 (1995). However, such techniques often result in the formation of hexagonal pyramids, commonly known as "hillocks," which are a manifestation of the natural lattice structure of III-V nitride materials. These hillocks are often viewed as defects unsuitable for use in any application because of their topology and crystalline structures. However, high-quality crystallites with a topology suitable for device fabrication have been described in recent publications such as in "Spatial Distribution of the Luminescence in GaN Thin Films," by Ponce et al., Appl. Phys. Lett. 68(1), pp. 57-59 (1996).